The present invention is directed to the field of continuous industrial furnaces used to heat metal billets or other separate pieces. A standard production furnace 10 is shown in FIGS. 1A and 1B. Units of product 12 are advanced through the furnace 10 along a movable hearth or beam 14. Burners 16 are fired into the furnace 10 so as to heat the product 12.
In a standard METAL Ras reheating application, it is typically desirable to heat a load to 2000-2400.degree. F. This heating is achieved by firing burners 16 sufficient in size and number to establish a furnace thermal environment having products of combustion (POC's) at a temperature of 2000-2500.degree. F. Burner flame temperatures are typically above 3000.degree. F. Thus, care must be taken to ensure that the burner flame does not directly impinge upon the product 12, which affects grain growth, surface properties and creates excessive "scaling" which reduces the quality and quantity of useful product output. To this end, it is common to install burners 16 near the top of the furnace walls so that they fire horizontally, i.e. parallel to the top of the product 12, or mount radiant flat flame burners in the roof of the furnace 10, so as to preclude flame impingement. In some furnace configurations, the burners 16 can be placed to fire below the load.
There are drawbacks associated with the standard furnace design. Since burner firing occurs above and/or below the load, there tends to be an uneven thermal distribution within the furnace chamber. A boundary layer 18 exists which separates the load from a region 20 of the hot, radiant POC's exiting the burners 16. The spaces between and around the billets of product 12 tend to retain pockets 22 of stagnant furnace gas that are much cooler than the hot POC's in hot region 20. Thus, most of the heat is transferred to the product 12 by radiation from above to the top surface and fractional AREAS between pieces of the product 12, thereby limiting the rates of heat transfer to the product 12. Thus, the product 12 must spend a longer time in the furnace 10 in order to obtain the desired heating effect, resulting in reduced throughput productivity and greater energy consumption.